Category: Experimental R. Chandra cisco Systems June 1996
BGP Route Reflection
An alternative to full mesh IBGP Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
The Border Gateway Protocol [1] is an inter-autonomous system routing protocol designed for TCP/IP internets. BGP deployments are
configured such that that all BGP speakers within a single AS must be fully meshed so that any external routing information must be re- distributed to all other routers within that AS. This represents a serious scaling problem that has been well documented with several alternatives proposed [2,3].
This document describes the use and design of a method known as "Route Reflection" to alleviate the the need for "full mesh" IBGP.
1. Introduction
Currently in the Internet, BGP deployments are configured such that that all BGP speakers within a single AS must be fully meshed and any external routing information must be re-distributed to all other routers within that AS. This "full mesh" requirement clearly does not scale when there are a large number of IBGP speakers as is common in many of todays internet networks.
For n BGP speakers within an AS you must maintain n*(n-1)/2 unique IBGP sessions. With finite resources in both bandwidth and router CPU this clearly does not scale.
This scaling problem has been well documented and a number of proposals have been made to alleviate this [2,3]. This document represents another alternative in alleviating the need for a "full mesh" and is known as "Route Reflection". It represents a change in the commonly understood concept of IBGP and the addition of two new
optional transitive BGP attributes.
2. Design Criteria
Route Reflection was designed to satisfy the following criteria.
o Simplicity
Any alternative must be both simple to configure as well as understand.
o Easy Migration
It must be possible to migrate from a full mesh
configuration without the need to change either topology or AS. This is an unfortunate management overhead of the technique proposed in [3].
o Compatibility
It must be possible for non compliant IBGP peers to continue be part of the original AS or domain without any loss of BGP routing information.
These criteria were motivated by operational experiences of a very large and topology rich network with many external connections.
3. Route Reflection
The basic idea of Route Reflection is very simple. Let us consider the simple example depicted in Figure 1 below.
+--- + +---+
| | IBGP | | | RTR-A |---| RTR-B | | | | | +---+ +---+
\ / IBGP \ ASX / IBGP \ /
+---+
| | | RTR-C | | | +---+
Figure 1: Full Mesh IBGP
In ASX there are three IBGP speakers (routers RTR-A, RTR-B and RTR- C). With the existing BGP model, if RTR-A receives an external route and it is selected as the best path it must advertise the external route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers) will not re-advertise these IBGP learned routes to other IBGP speakers.
If this rule is relaxed and RTR-C is allowed to reflect IBGP learned routes, then it could re-advertise (or reflect) the IBGP routes learned from RTR-A to RTR-B and vice versa. This would eliminate the need for the IBGP session between RTR-A and RTR-B as shown in Figure 2 below.
+--- + +---+
| | | | | RTR-A | | RTR-B | | | | | +---+ +---+
\ / IBGP \ ASX / IBGP \ /
+---+
| | | RTR-C | | | +---+
Figure 2: Route Reflection IBGP
The Route Reflection scheme is based upon this basic principle.
4. Terminology and Concepts
We use the term "Route Reflector" (RR) to represent an IBGP speaker that participates in the reflection. The internal peers of a RR are divided into two groups:
1) Client Peers 2) Non-Client Peers
A RR reflects routes between these groups. A RR along with its
client peers form a Cluster. The Non-Client peer must be fully meshed but the Client peers need not be fully meshed. The Client peers
should not peer with internal speakers outside of their cluster.
Figure 3 depicts a simple example outlining the basic RR components using the terminology noted above.
/ - - - -\
| Cluster | +---+ +---+
| | | | | | | RTR-A | | RTR-B | | |Client | |Client | | +---+ +---+
| \ / | IBGP \ / IBGP | \ / | +---+
| | | | | RTR-C |
| | RR | | +---+
| / \ | \ - - - - -/- - -\- - - / IBGP / \ IBGP
+---+ +---+
| RTR-D | IBGP | RTR-E | | Non- |---| Non- | |Client | |Client | +---+ +---+
Figure 3: RR Components 5. Operation
When a route is received by a RR, it selects the best path based on its path selection rule. After the best path is selected, it must do the following depending on the type of the peer it is receiving the best path from:
1) A Route from a Non-Client peer Reflect to all other Clients.
2) A Route from a Client peer
Reflect to all the Non-Client peers and also to the Client peers other than the originator. (Hence the Client peers are not required to be fully meshed).
3) Route from an EBGP peer
Send to all the Client and Non-Client Peers.
An Autonomous System could have many RRs. A RR treats other RRs just like any other internal BGP speakers. A RR could be configured to have other RRs in a Client group or Non-client group.
In a simple configuration the backbone could be divided into many clusters. Each RR would be configured with other RRs as Non-Client peers (thus all the RRs will be fully meshed.). The Clients will be configured to maintain IBGP session only with the RR in their
cluster. Due to route reflection, all the IBGP speakers will receive reflected routing information.
It is normal in a Autonomous System to have BGP speakers that do not understand the concept of Route-Reflectors (let us call them
conventional BGP speakers). The Route-Reflector Scheme allows such conventional BGP speakers to co-exist. Conventional BGP speakers ould be either members of a Non-Client group or a Client group. This
allows for an easy and gradual migration from the current IBGP model to the Route Reflection model. One could start creating clusters by configuring a single router as the designated RR and configuring other RRs and their clients as normal IBGP peers. Additional clusters can be created gradually.
6. Redundant RRs
Usually a cluster of clients will have a single RR. In that case, the cluster will be identified by the ROUTER_ID of the RR. However, this represents a single point of failure so to make it possible to have multiple RRs in the same cluster, all RRs in the same cluster must be configured with a 4-byte CLUSTER_ID so that an RR can discern routes from other RRs in the same cluster.
7. Avoiding Routing Information Loops
As IBGP learned routes are reflected, it is possible through mis- configuration to form route re-distribution loops. The Route Reflection method defines the following attributes to detect and avoid routing information loops.
ORIGINATOR_ID
ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type code 9. This attribute is 4 bytes long and it will be created by a RR. This attribute will carry the ROUTER_ID of the originator of the route in the local AS. A BGP speaker should not create an
ORIGINATOR_ID attribute if one already exists. A route reflector must never send routing information back to the router specified in ORIGINATOR_ID.
CLUSTER_LIST
Cluster-list is a new optional, non-transitive BGP attribute of Type code 10. It is a sequence of CLUSTER_ID values representing the reflection path that the route has passed. It is encoded as follows:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| Length | value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where Length is the number of octets.
When a RR reflects a route from its Clients to a Non-Client peer, it must append the local CLUSTER_ID to the CLUSTER_LIST. If the
CLUSTER_LIST is empty, it must create a new one. Using this attribute an RR can identify if the routing information is looped back to the same cluster due to mis-configuration. If the local CLUSTER_ID is found in the cluster-list, the advertisement will be ignored.
8. Implementation and Configuration Considerations
Care should be taken to make sure that none of the BGP path
attributes defined above can be modified through configuration when exchanging internal routing information between RRs and Clients and Non-Clients. This could result is looping of routes.
In some implementations, modification of the BGP path attribute, NEXT_HOP is possible. For example, there could be a need for a RR to modify NEXT_HOP for EBGP learned routes sent to its internal peers.
However, it must not be possible for an RR to set on reflected IBGP routes as this breaks the basic principle of Route Reflection and will result in potential black holeing of traffic.
An RR should not modify any AS-PATH attributes (i.e. LOCAL_PREF, MED, DPA)that could change consistent route selection. This could result in potential loops.
The BGP protocol provides no way for a Client to identify itself dynamically as a Client to an RR configured BGP speaker and the simplest way to achieve this is by manual configuration.
9. Security Considerations
Security issues are not discussed in this memo.
10. Acknowledgments
The authors would like to thank Dennis Ferguson, Enke Chen, John Scudder, Paul Traina and Tony Li for the many discussions resulting in this work. This idea was developed from an earlier discussion between Tony Li and Dimitri Haskin.
11. References
[1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995.
[2] Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh routing", RFC 1863, October 1995.
[3] Traina, P., "Limited Autonomous System Confederations for BGP", RFC 1965, June 1996.
12. Authors’ Addresses Tony Bates
cisco Systems
170 West Tasman Drive San Jose, CA 95134 Phone: +1 408 527 2470 EMail: tbates@cisco.com
Ravishanker Chandrasekeran (Ravi Chandra)
cisco Systems
170 West Tasman Drive San Jose, CA 95134
EMail: rchandra@cisco.com
